Recently, with an increasing number of various digital copying machines widely spread, systems capable of connecting various interface devices to the digital copying machines and outputting image data from the host computer have been developed.
However, since these systems sequentially interpret input document data when a document referring to external data is output, and obtain the external data when an external data reference instruction is issued, an output data process is suspended until an external data obtaining process has been completed, thereby reducing the throughput of the entire system.
Additionally, since these systems sequentially interpret input document data when a document referring to external data is output, and if an external data reference instruction is actually issued, obtain the external data each time the external data reference instruction is actually issued, data can be wastefully obtained when a plurality of documents refer to the same external data.
Furthermore, when external data is obtained in the same job, and when it takes a long time to obtain data due to the delay in a network, etc. the data is waited without processing other jobs or pages, thereby reducing the throughput of the entire system.